Project Summary The goal of this project is to investigate the postnatal role of Pitx2 in atrial fibrillation. Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, a class of diseases characterized by abnormal electrical impulse conduction throughout the heart. AF is a disease of aging, currently affecting more than three million Americans, a number estimated to double as the populace ages. While the genetic contributions to AF are still being investigated, multiple genome wide association studies have demonstrated the presence of an AF- associated region on chromosome 4q25, with Pitx2 being the closest gene to the region. Pitx2 codes for an essential transcription factor that is critical in forming left-right asymmetry present in the heart and other organs. Following development, Pitx2 expression is restricted largely to left atrial cardiomyocytes, where its continued function is not well characterized. Recent work from our lab has demonstrated that Pitx2 expression is induced in postnatal ventricular cardiomyocytes following injury, where it transcriptionally regulates the oxidative stress response. Multiple genetic mouse models have demonstrated that both developmental and postnatal reductions in Pitx2 expression are sufficient to cause atrial arrhythmias; however, the mechanism behind these results are largely unknown. Our long-term goal is to gain insight into the role of Pitx2 in the postnatal left atrium, with the hopes of developing novel therapeutics focused on treating this subpopulation of AF patients. We have preliminary data demonstrating that there is a significant shift in the transcriptional profile of left atrial fibroblasts following postnatal deletion of Pitx2 from cardiomyocytes in our genetic mouse model of AF. We hypothesize that postnatal reductions in Pitx2 increase reactive oxygen species activity in left atrial cardiomyocytes and promotes nonautonomous fibroblast activation, resulting in atrial fibrillation. To test this hypothesis, we will determine the mechanism of cardiac fibroblast activation following Pitx2 deletion using mouse genetics. We will then determine the relationship between fibroblast activation and arrhythmia formation. Together, the proposed studies will lend insight into the function of Pitx2 in postnatal cardiomyocytes and help us achieve our goal of developing novel therapeutics for the treatment of AF.